1. Field of the Invention
The present invention generally relates to a composition and method in use for surface treatment of parts made of Al Die Casting material (hereinafter will be referred to as “ALDC material”). In particular, the invention pertains to a surface treatment composition and method without generation of nitrogen oxide (NOx) or hydrogen fluoride (HF) for removing Si and reduced metal salt produced during etching of the ALDC material.
2. Description of the Prior Art
An ALDC material generally contains Al and other ingredients such as Si, Fe, Cu, Mn, Mg, Zn and Ni. Such ALDC material requires surface treatment such as plating, painting and anodizing in order to improve corrosion resistance, and appearance. In general, the ALDC material undergoes a surface treatment including plating, process which is carried out in the order of degreasing, etching, desmutting, zincating and plating, and painting process which is carried out in the order of degreasing, etching, desmutting, drying and painting.
In the above processes, etching is generally carried out in a NaOH aqueous solution. Etching for the ALDC plating is performed at a temperature ranging from a room temperature to 50° C. in an about 5 to 20% NaOH aqueous solution in order to remove any oxide layer and/or oil from the surface of the ALDC, and closely related to formation of surface roughness for plating.ALDC (1)→ALDC (2)+Al dissolution {3NaOH+Al→Al(OH)3+H2↑} (NaOH aqueous solution, etching)
In the above process, components such as Si, Fe, Cu, In, Mg, Zn and Ni contained in ALDC (1) appear on the surface of the ALDC (2) as Al, that is the main component of the ALDC, are dissolved during etching, in which those components such as Si, Cu, Fe, Mn and Ni are not dissolved into the NaOH aqueous solution and accordingly remain in the surface of ALDC (2).
Tables 1A and 1B and FIGS. 1A and 1B respectively represent results of EDAX analysis about the components in the surface of ALDC (2) which has been dipped for 10 minutes in a 10% NaOH aqueous solution, in which the ALDC materials are analyzed based upon ALDC-7 and ALDC-8.
TABLE 1AAnalysis of Surface Component in ALDC-7 (2)Result of Quantitative AnalysisComponentWeight %Atomic %O14.9126.00Cu7.963.49Na1.782.16Mg0.000.00Al6.636.85Si54.9954.63Fe13.756.87
TABLE 1BAnalysis of Surface Component in ALDC-8 (2)Result of Quantitative AnalysisComponentWeight %Atomic %O17.0530.11Cu4.832.15Na2.513.09Mg1.772.06Al20.8521.83Si27.9198.08Fe25.0712.68
As can be seen in Tables 1A and 1B, those components such as Si, Fe and Cu are the most prominent components after etching of the ALDC material. Those components should be necessarily removed from the surface of the materials since they influence coherence and uniformity during final plating, painting and anodizing.
In an aqueous solution, generally each of the metal components is reduced and deposits to the surface of an aluminum material such as Al or Al alloy (Al 2024, Al 5083) so as to form reduced metal salts during oxidation reaction, in which the etched Al or Al alloy material is mainly dipped into a HNO3 aqueous solution according to a conventional method for removing the reduced metal salts.
When the ALDC material is dipped into the HNO3 aqueous solution, the metal components such as Cu, Fe and Ni are dissolved and ionized in the HNO3 aqueous solution as follows:Cu→Cu2++2e-;Fe→Fe2++2e-; andNi→Ni2++2e-.
On the other hand, a mixed acid solution of HNO3 and HF has been conventionally used to remove the metal and Si components formed on the material during NaOH etching of the ALDC material. This has a reaction mechanism as follows:    (1) Reaction mechanism of metal components except for Si contained in ALDC with HNO3:Me (Fe, Cu. Mn, Mg, Zn, Ni)+HNO3+H2O→MeO+HNO2+H2O→MeO+H++NO2↑+H2O (First Reaction)→MeO+2HNO3+H2O→Me(NO3)2+H2O (Second Reaction)
In the first reaction, HNO2 is gasified into H+ and NO2 in the aqueous solution and thus emits toxic NOx gas, which is harmful to humans and the environment.
(2) Reaction mechanism of SiO2 with HF:SiO2+4HF→SiF4+2H2O+H2↑ (First Reaction)2HF+SiF4→H2SiF6 (Second Reaction)
In the first reaction, HF gas is emitted owing to generation of H2 gas.
Although a chemical treatment method of the ALDC material using HNO3 and HF has been performed for a long time, this method produces a large amount of NOx and HF gases and thus fatally acts to human bodies and facilities.
FIG.2 illustrates HF and NOx gases which are produced from the ALDC material treatment with a mixed acid of HNO3 and HF, and Table 2 represents measured amounts of the HF and NOx gases. As can be seen in Table 2, mixed acid of HNO3 and HF emits the NOx and HF gases by a large amount thereby representing a severe obstacle against substitution of the ALDC for Al.
TABLE 2Quantity of HF and NOx Emission during ALDC material Treatment inMixed Acid of HNO3 and HFNOxHFQuantity of Gas Generation10,722.223,840.53(ppm)
Also as known in the conventional art, methods have been used to clean foreign materials from semiconductor board by using compositions each composed of an inorganic substance containing hydrogen peroxide and fluorine ion as disclosed in Japanese Patent Laid-Open Nos. H08-250461 and H10-298589. However, the compositions show poor oxidizing power owing to very small value of peroxide content. The above substrate-cleaning agents may partially remove Si from the surface of the ALDC material during etching but fails to simultaneously remove the reduced metal components such as Fe, Cu, Mn, Mg, Zn and Ni from the same.